This proposal describes studies of the control of human orofacial movement that focus on the jaw. The goal is to understand the ways in which neural signals interact with orofacial biomechanics to determine movement outcomes in speech. All of the proposed research is related by the common aim of understanding the relationship between neural and biomechanical determinants of movements. The studies extend from the jaw's biomechanical properties and their relation to speech variability to the degree to which control signals compensate for the dynamics of the mechanical apparatus. A common procedure is adopted throughout - the use of a servo-controlled robotic manipulator to deliver precise mechanical perturbations to the jaw. The research will (1) document the jaw's biomechanical properties and in particular the relationship between jaw stiffness in statics and kinematic variability during vowel production. (2) The work will characterize jaw stiffness during movement and assess the dependence of stiffness on movement speed, evaluate stiffness in speech versus mastication, and determine the relationship between stiffness over the course of movement and time-varying patterns of kinematic variability. (3) The studies will also assess the complexity of the information that the nervous system encodes concerning the biomechanical periphery and external loads in orofacial motion planning.